Damascene processing is a method for forming interconnections on integrated circuits that involves formation of inlaid metal lines in trenches and vias formed in a dielectric layer (inter-metal dielectric). The metal conductive lines are formed by an electroplating process. Because copper or other mobile conductive material provides the conductive paths of the integrated circuit, the underlying silicon devices must be protected from metal ions (e.g., Cu2+) that might otherwise diffuse or drift into the silicon. Suitable materials for diffusion barrier include tantalum, tantalum nitride, tungsten, titanium tungsten, titanium nitride, tungsten nitride, and the like.
After diffusion barrier is deposited and prior to electroplating, a seed layer of copper or other metal is typically applied by a physical vapor deposition (PVD) process to enable subsequent electrofilling of the features with copper inlay. In order to serve as a seed for electroplating, the seed layer should be continuous, stable and have good adhesion to the barrier layer.
One difficulty with depositing seed layers is that the copper may agglomerate on the barrier layer surface. This phenomenon occurs because of the weak chemical bond between seed and the barrier as a result of the absence of an intermetallic reaction between the two. Because of this agglomeration, the copper may not cover the surface in a continuous manner, especially on the sidewalls of the recessed features. The thickness of the seed layer coverage is thus uneven, thicker in some places than others, and the layer may include gaps. Such seed layers do not provide a uniform layer for electroplating, which leads to voiding defects in the electroplated copper. Conventionally, in order to avoid discontinuity of seed layers, large amounts of seed layer material had to be deposited.